1. Field of the Invention
The present invention relates to a focus adjustment apparatus and a focus adjustment method and, more particularly, to a focus adjustment apparatus and a focus adjustment method for performing a focus adjustment by using pixel signals based on object light which passes through different exit pupils of a focusing optical system, and to a storage medium in which a focus adjustment program has been stored and an imaging apparatus.
2. Description of the Related Art
As one of focus adjustment methods of an imaging apparatus, there is an imaging plane phase difference method of performing a focus detection of a phase difference method with focus detection pixels formed on an imaging element. In a focus adjustment of the imaging plane phase difference method, a defocus direction and a defocus amount can be simultaneously detected by the focus detection pixels formed on the imaging element and the focus adjustment can be performed at a high speed.
For example, U.S. Pat. No. 4,410,804 discloses an imaging apparatus using a two-dimensional imaging element in which one microlens and a plurality of divided photoelectric conversion units are formed with respect to one pixel. The divided photoelectric conversion units are constructed so as to receive rays of light from different areas of an exit pupil of a photographing lens through one microlens, and attain a pupil division. A correlation amount is calculated from focus detection signals received by the divided photoelectric conversion units (focus detection pixels), an image shift amount is obtained from the correlation amount, and the focus detection of the phase difference method can be performed. The Official Gazette of Japanese Patent Application Laid-Open No. 2001-083407 discloses such a technique that an image pickup signal is generated by adding focus detection signals received by the divided photoelectric conversion units.
The Official Gazette of Japanese Patent Application Laid-Open No. 2000-156823 discloses such an imaging apparatus that a pair of focus detection pixels are partially arranged to a two-dimensional imaging element constructed by a plurality of image pickup pixels. In such an imaging apparatus, the pupil division is performed by constructing one set of focus detection pixels in such a manner that object light which passes through different areas of an exit pupil of a photographing lens is received by a light shielding layer having an aperture portion. There is also disclosed such a technique that image pickup signals are obtained by the image pickup pixels arranged in most of the two-dimensional imaging element, a correlation amount is calculated from pixel signals of the focus detection pixels which are partially arranged, and an image shift amount is obtained from the correlation amount, thereby performing the focus detection of the phase difference method.
However, according to the focus detection of the imaging plane phase difference method, since the pupil division is attained by using the microlens of the imaging element, the focus detection is affected by diffraction, so that a pupil division area is not a distinct area but appears as light receiving ratio distribution and a frame vignetting occurs by a lens frame or an iris frame of the photographing lens. When the frame vignetting occurs, an agreement of form between a pair of focus detection signals which are used for the phase difference deteriorates, a distortion is emphasized by a differential filter which emphasizes an edge, and there is such a problem that an erroneous in-focus detection (false in-focus detection) happens.